464,664
ee 6 4 ,
AMSTRAD 464, 664 & 6128
Waerzoys
AMSTRAD 464, 664 & 6128
I.KALISKY & D.KEDEM
UNIT 3 COMPUTER GAMES
GLENTOP
PUBLISHERS © LIMITED
DECEMBER 1985
All programs in this book have been written expressly to illustrate specific teaching points. They are not warranted as being suitable for any particular application. Every care has been taken in the writing and presentation of this book but no responsibility is assumed by the author or publishers for any errors or omissions contained herein.
COPYRIGHT ° Glentop Publishers Ltd 1985 Translated from the original. COPYRIGHT ° |. Kalisky & D. Kedem 1984, 1985
No part of this publication may be copied, transmitted or stored ina retrieval system or reproduced in any way including but not limited to photography, photocopy, magnetic or other recording means, without prior permission from the pubfishers, with the exception of material entered and executed on a computer system for the reader’s own use.
ISBN 1 85181 005 6
Published by:
Glentop Publishers Ltd Standfast House
Bath Place
High Street
Barnet
Herts EN5 1ED
Tel: 091-441-4130
Cover illustration from an original painting by Nick Woods.
Contents
Foreword
Chapter 1 - Video Pencil Chapter 2 - Shooting the Aliens Chapter 3 - Number Games Chapter 4 - A Horserace Game Chapter 5 - String Games Chapter 6 - A Treasure Hunt Summary Index
Answers
Page
19
27
39
47
57
61
63
Foreword
In this unit, we will concentrate on developing computer video games and in doing so, learn more about Amstrad BASIC.
The techniques taught during this unit are the basic background to writing all computer video games, however advanced they may be.
By the end of the unit, you will have improved you knowledge of the BASIC language considerably.
Itzhak Kalisky and Dani Kedem.
Chapter 1 VIDEO PENCIL
In the first chapter of this unit, we will develop
a ‘video pencil’ program which is rather like a Video Pencil computer ‘etcha sketch’ and should work in the
following way:
@ When you run the program, an asterisk is displayed in the centre of the screen.
RUN
e If you press the letter ‘K’, the asterisk moves to the right, leaving the previous ( |
asterisks on the screen.
@ If you press the letter ‘M’, the asterisk moves down the screen until you take your finger off the key.
me M * * elif you press ‘J’, the asterisk moves to the left and if you press ‘I’, the asterisk moves up the screen. | AMSTRAD 464 >} acy poooont IOOOUOOOE I (el rInooououUEsoooo | alele ( JUDUOOOm OOOO OoOu0 ( JO00000sUOOoOC | § joou = ee elele)
(The diagram above is of an Amstrad 464 computer which we have chosen to use to show the various key positions. This shouldn’t cause any problems for those of you who have 664 or 6128 models.)
Note: we have chosen lI, J, K and M as the direction keys, however, we could have chosen any four keys.
Before you can write the video pencil program, you must know the following command.
IF...THEN
ty) ~ Do you know what the following program does?
19 INPUT A 20 IF A=5 THEN PRINT "BOOM" 39 GOTO 19
O When you think you know what the program does, type it in and run it.
a Input the numbers from 1 to 19, remembering to press ENTER after each one.
ae,
t What happens? Explanation
When the computer executes line 2Q, it checks to see if A equals 5.
®@ If it doesn’t, the computer ignores line 20 and carries on to line 39.
@ = If the condition exists, ie. you entered 5, the computer prints ‘BOOM’.
Before we continue with the video pencil program, lets just look at the IF...THEN command a bit more.
Task
Write a program to perform the following:
@ The computer waits for the input of a number from 1 to 1@ and on entering a number, eg. 1, the computer prints ‘ONE’ on the screen.
@ When you enter the number 2, the computer prints ‘TWO’. Answer 1
Task
Write a program to do the following:
@® On pressing the first letter of your name, the computer displays your name in full.
@ = sIf you press the letter ‘L’, the computer displays the whole program.
@ If you press the letter ‘N’, the program is erased from memory (NEW). Answer 2
Games Corner
CL Write a program to create the following game which you and a friend can play.
@ Tell your friend to leave the room for a moment whilst you run the program. At this point, you enter a ‘secret’ number (from 1 to 99) into the computer which the program then erases from the screen so your friend can’t see it.
@® Your friend then returns to the room and tries to guess the secret number.
10
@® = If he/she guesses ‘higher’ than the secret number, the computer prints ‘Too high’.
@ If the guess is ‘lower’ than the secret number, the computer prints ‘Too low’.
® If the guess is correct, the computer informs your friend appropriately.
When your friend has guessed the number, it is your turn to go out of the room.
The aim of the game is to guess the secret number in as few guesses as possible.
Hint: IF A>B Greater Than
means if Ais GREATER THAN B and
IF A<B Less Than means if Ais LESS THAN B. [AMSTRAD 464. oI c Iq HooooooooooOooo! C) TIODOOOOOUUOUOUE 10 aooooo00o0ooo0ooL]) §6jgoo (JIDOUOODOmEoOoOC_ |ooo {SSS hf OO
Back to the Video Pencil
Now you know the IF...THEN command, we can go back to the Video Pencil program .
If your not sure how the IF...THEN command can be utilized in the video pencil program, look at the following lines:
IF A$="K" THEN X=X+1
LOCATE X,Y:PRINT "*" Answer 3
11
Although the program works, it is slow because you have to press ENTER after each direction key.
Improving the Video Pencil
To make the program work according to the description on page 7, i.e. without pressing ENTER, we have to learn a new command.
INKEY$
Instead of having INPUT A$ in the program, put:
AS=INKEY$
Note: if you used a different variable name eg. INPUT C$, the line will become C$=INKEY$. INKEY$
OO Now run the program and move the asterisk around the screen. You can see that you don’t have to press ENTER anymore.
Explanation
It’s obvious that this change came about from the inclusion of the INKEY$ command, but, what does INKEY$ do?
When the computer executes an INKEY$ command, nothing happens until you type ina character. The character input is then stored in the variable specified.
Note: INKEY$ only accepts one character whereas INPUT accepts upto 255 characters (String INPUT).
Therefore, when the computer executes:
AS=INKEY$
it puts the character input into the variable A$, without having to press ENTER.
12
For example:
A$=INKEY$ AMSTRAD 464. | day OoOooooooooooooI alin COOOOOUOmoOoL ol oou Todo i Igo = ooo Tas | IDOOUOOOOLI ] o000 pe le oF looo AS
In this case, A$ is equal to (I, J, K, M)? The computer then displays an asterisk above the previous one.
When the computer executes A$=INKEY$ again, it puts the current character pressed into A$ and continues as before.
Restricting the Pencil
You’ve probably noticed that when the ’pencil’ goes over the edge of the screen, the computer reacts in a strange way.
It would be a better program if you stopped the user doing this.
C Therefore, add lines to the program so that the pencil cannot go over the edges of the screen. Answer 4
ry What do you think would happen if you changed line 80 to PRINT A$ instead of PRINT
64D
Now press CAPS LOCK and run the program. Why doesn’t the pencil move? Answer 5
13
More on INKEY$
ry What would happen if you didn’t input a character when the computer executed an INKEY$ command?
The computer would simply put a Null String into the variable A$. (If you don’t remember what a Null String is, look at Unit 2, Chapter 4.)
In this case, A$ would receive a value of (ae hie ee la “K” “M”)?
The important thing to remember is that INKEY$ will always receive a value whether it be a character or a null string. Therefore, the program will not wait for input, like it does with the INPUT command.
J 7 How can we prove this? Answer 6
Another way of proving that the computer
continues to run is by producing a short sound
each time the computer executes the INKEY$
command. (If you don’t remember how to
produce sound, look at Unit 1, Chapter 7.) Answer 7
You will hear a continuous sound, which means that the INKEY$ is executed repeatedly. If it was only executed once, the sound would have stopped.
Before going on, delete line 35.
Comparing INPUT and INKEY$
The INPUT and INKEY$ commands are both ways of defining variables, however they do differ:
e With INKEY$, you do not have to press ENTER.
14
r INKEY$ only accepts one character whereas INPUT accepts upto 255 characters.
@ INKEY$ does not wait for a character to be input. ie. it accepts null characters.
Changing the Pencil
CW) Add lines to the program so that when you press a certain key (eg. “}”), the computer goes to a part of the program which allows you to change the pencil character, which at present is a ‘*’.
When you have changed the pencil, the computer continues drawing from the last point displayed with the previous pencil.
Hint: what do the following lines do:
150 BS=INKEY$ 160 IF B$="" THEN GOTO 159
OO Add these lines to the program and run them (RUN 159).
UL Now try to complete the program on your own. Answer 8
Task
Change the video pencil program so that
instead of using the existing keys (I, J, K, and
M), the four cursor keys are used. [AMSTRAD 464 ul f ta
JOoOOoOoOoOoOoOoOooOooe I ia 100 nanan | : =]
OO} eS
Hint
In Unit 2, Chapter 5, we saw how each character had a character number (CHR$).
15
| vr, What are the character numbers of the four cursor keys (*,+,4,4)? Answer 9
Now, all you have to do to solve this task is to substitute the previous keys with these new character values. Answer 10
Coloured Pencil
& Modify the program so that you can
change the colour of the pencil at any time. Answer 11 Task Using the video pencil, write your name on the screen. PPPP AAAA U U L P PAA U UCL PPPP AAAA U U L P AAU UL P A A UUUU LLLL
At this stage, the task is difficult because we need to move the pencil without printing anything. eg. from the ‘P’ to the ‘A’. This can be done by either using a ‘space pencil’ (B$=“ ”) or by setting the colour of the pencil to the same colour as the screen (C=4), however, you cannot be sure of the pencil’s position.
CU Therefore, change the program so that when you press the ‘~’ key, the pencil moves as before but it doesn’t leave the characters on the screen.
PPPP L1L1C) .........AAAA
Here you press "—" (LC) means space)
When you want to continue drawing, press the ‘=’ key and the pencil should return to its original state. Answer 12
16
Joysticks
For those of you who have joysticks, we will Joysticks show you how to use them in the video pencil
program instead of having to use the cursor
keys.
Firstly, you must connect your joystick to the Amstrad making sure that it is not on ‘Autofire’. If it is, all sorts of things will happen to the computer.
JOY(9)
C) Add the following lines to the program in memory and run them:
590 CLS 518 LOCATE 26,19:PRINT JOY(9) JOY(®) 526 GO T0 519
You can see that the number @ appears in the middle of the screen.
v7 What happens if you move the joystick forward?
_ r What happens when you centre the joystick again?
Cc Move the joystick in every possible direction and see how the numbers change according to its position.
Explanation
In line 51@, you can see that we PRINT JOY(Q). JOY(Q@) is a function that returns the status of the joystick. ie. it displays a different value for each joystick position.
JOY(@)
17
When the joystick is centred, JOY(@) returns the value of (1, 0)?
When the joystick is forward, JOY(Q) returns the value of (1, 0)?
? Fill in the values that JOY(@) returns when the joystick is in each of the following positions: Answer 13
Up
Left Right
Down
Note
When the computer prints JOY(Q), the current position of the joystick is displayed and the program continues to run.
5) § What value does JOY(Q) return when you press the fire button on the joystick?
£ And what about when you move the joystick and press fire? Answer 14
i
CL Now you know about joysticks, try to
incorporate the joystick into the video pencil
program. Answer 15
Improving the Program
At present, the video pencil can move in four directions. Change the program so that it can move in every direction that the joystick can
move in. ie. diagonally.
18
Chapter 2
SHOOTING THE ALIENS
We think that at some point in time, most of you must have played a video game where you have had to shoot invaders or runaway from monsters.
In this chapter, we will construct a video game called ‘Shooting the Aliens’ which will work as follows:
@ The ‘alien spaceship’ (>) will continuously move across the screen (from left to right). Your spaceship ($) waits below.
@ When the alien spaceship is directly above your spaceship, press ‘fire’. (Fire can be any key you wish but it should be easy to reach. ie. space bar.)
@ At this point, the alien spaceship stops and the missile (¢) is launched.
The aim of the game is to hit the alien spaceship.
19
20
Let’s see if you know all you need to know to write this program:
@ Do you know how to move the alien spaceship across the screen? (Yes or No).
® Do you know how to display your spaceship at the bottom of the screen? (Yes or No).
@ Do you know how to detect when the fire button has been pressed? (Yes or No).
@ Do you know how to move the missile? (Yes or No).
The answer to all these questions should be ‘yes’ meaning you can start writing the program.
Stage 1
The first stage is to move the alien spaceship across the screen continuously and place your spaceship at the bottom of the screen. Answer 16
Stage 2
You must now choose a key to act as the fire button. We will assume that you chose the space bar as the fire button.
The alien spaceship continuously moves across the screen until you press the spacebar (fire). When you press fire, the computer launches its missile. —
21
Below is a diagram which illustrates how the program should be constructed:
The alien spaceship continuously moves
The missile is launched.
Note: use the INKEY$ command to implement the fire button. ie. you don’t have to press ENTER after pressing the spacebar. Answer 17
Stage 3
Once you have pressed fire, the missile needs to be launched. What we mean by ‘launch’ is, move the missile up the screen.
(Use a negative STEP value.) Answer 18
You have now finished the basic structure of the game, however, improvements can always be made.
Space Sounds
A video game is not a video game without sound. Add sound to the program to accompany the alien spaceship’s movement and the firing of the missile.
(If you don’t remember how to use the SOUND command, look at Unit 1, Chapter 7 and Unit 2, Chapter 1.)
22
Task
Change Stage 1 of the program so that as the alien spaceship moves across the screen, a sound is produced whose pitch gets lower and lower.
Hint
The pitch should start at about 5@ and be incremented by the loop variable X.
Note: make sure you get rid of the delay first. Answer 19
A Strange Reaction
= ¢ What happens to the alien spaceship and the sound produced when you first run the program?
ty & What happens to the sound immediately after pressing fire?
CL Run the program and stop it when the alien spaceship is in motion.
‘@ Now instruct the computer to:
SOUND 1,390,109
When you press ENTER, the computer continues to sound the notes within the program.
At this stage, we won’t explain why the sound doesn’t immediately stop when you press fire, but if you add the following line to the program, it will.
57 IF SQ(1)>127 THEN GOTO 57
_ After you’ve added the line, run the program.
23
Task
Modify the program so that when you press fire, a sound whose pitch rises, is produced. Answer 20
Sound Default
As you may remember, the third number in the SOUND command specifies the length of the note produced.
7 What happens if you leave out the third number? For example:
SOUND 1,X+59
CL Take out the third number in line 55 and run the program.
7 Why does each note now take longer to play?
If you leave out the third number in the SOUND command, the note played will take the default value as the length. If something has a default value, it means that it has already been specified by the computer. In this case, the default value for the length of a sound is 20.
v7) Does the second number in the SOUND command have a default value?
Check this by excluding the second number from the command.
Summary
It is only the third number in the SOUND command that has a default value and this is equal to ____.
24
Improving the Program
e Add lines to the program so that the game continues even after you have fired the first missile.
Note: if you miss the alien spaceship, remember to erase it from the screen before continuing onto the next alien.
e Display the amount of missiles you have fired.
Hint: you need to include a section which counts. For example, the watch program in Unit 2, Chapter 3.
@ Instruct the computer to display the amount of hits you have had.
If you don’t know how to do this, answer the following questions:
&)
§ In which column (X) must the alien spaceship be in order for the missile to hit it? (5, 10, 35)?
G5)
& Assuming you have chosen M as the count, which of the following lines will solve the problem:
LET M=M+1 IF X=35 THEN LET M=M+1
IF Y=35 THEN LET M=M+1
Answer 21
Answer 22
35
Answer 23
25
CU Now insert the appropriate lines into the program. Answer 24
@ Stop the program running when the user has fired 1@ missiles.
(Use the END instruction.) Answer 25
There are times when you only want to see
certain parts of a program rather than the
whole program. The LIST command can be Part LIST used to display certain specified lines. For
example:
LIST -109 will display all the lines upto line 10.
LIST 199-200
will display all the lines between line 19@ and line 200.
LIST 100 will display line 10@. @ Before you start the game, the computer
asks you at what speed you want the aliens to move at.
@ If you have a joystick, use its fire button instead of the spacebar.
(If you don’t know how to, look at page 16.)
To End the Game
Add lines to the program so that when you have mastered the game, ie. 10 out of 10 hits, the computer erases the program from memory (NEW).
26
27
Chapter 3 NUMBER GAMES
The Electronic Die
In this chapter, we will write a program that, on the press of a key, displays a number between 1 and 6 inclusively, like a die.
We will construct the die program stage by
stage:
CC) Firstly, run the following program:
18 PRINT RND RND 20 GOTO 19
) v What does the RND command do?
It tells the computer to RaNDomly choose a number. ie. the number displayed is not related to any other number.
? The numbers displayed on the screen are greater than or equal to (9.1, , 0.5) and less than (1, 0.9, 2).
Note: do you remember what the following number represents:
5.136E-93
This is another way of writing the decimal fraction 0.005136.
Run the program and check you answers.
28
Summary
From running the program, you can see that the RND command chooses numbers randomly between @ and 1 (but never 1).
CL Change the program so that the computer displays numbers between @ and 6, remembering that a die has 6 sides.
<>
18 PRINT RND*
20 GOTO 10 Answer 26 5) & Does the program display numbers
between @ and 6? It does, but a die never displays fractions. Therefore, we must somehow get rid of the fractions. The following program will do this:
1Q PRINT INT(RND*6)
29 GOTO 19 INT The INT command stands for INTeger and an INT
integer is a whole number. ie. a number that has no fraction. It is used in the following way:
INT(4.6)=4
Basically, all the INT command does is ‘round down’ the number in brackets. For example, 4.6 must be made a whole number. Usually in maths, you take the whole number that it is closest to. In this case, it would be 5, however the INT command takes the ‘lowest’ whole number. Therefore, 4.6 becomes 4.
29
rv} What would be the result of the following: INT(7.99) Think about it and then type in:
PRINT INT(7.99)
UL) Now run the program again and answer the following questions:
2 The number @ is displayed. Does this number appear on a real die? (Yes or No)?
= e The number 6 is not displayed. Does this number appear on a real die? (Yes or No)?
In other words, the computer is choosing numbers from @ to 5 instead of 1 to 6.
C Change the program so that numbers between 1 and 6 are displayed:
19 PRINT INT(RND*6)+ 20 GOTO 19 Answer 27
Football Pools
a Write a program that will fill in a football pool form, where:
0=Draw l=Team 1 wins 2=Team 2 wins Answer 28
30
Using Two Dice
Write a program so that when you press a key, the computer displays the result of throwing two dice.
® Draw acoloured box around each number.
@ Inform the user when there is a double. eg. play a tune or change the screens colour.
A Very Random Program
C) Write a program that displays stars (*) in random positions, in MODE @:
r) The computer displays the stars in random colours and produces random sounds for each star displayed.
(The pitch of the sound produced should be between 5@ and 259.)
Games Corner
The computer displays a submarine on the screen which consists of 5 asterisks in a line (*****), You are then asked how many bombs you want to drop on it.
lf, for example, you chose 4@ bombs, the computer randomly displays 40 bombs (O) on the screen.
If the bombs miss the submarine, you get 40 points. If it is hit (*O***), you don’t get any points.
Answer 29
KK KKK
31
You can play this game with a friend (each
having alternate turns) or with the computer
where the computer chooses for itself, the
number of bombs it is prepared to endanger its
submarine with. Answer 30
Improving the Game
Let’s assume that you want the computer to count both player’s score. For the computer to do this, it must know when the submarine has been hit.
vr) How will the computer know this?
AND
For the moment, let’s say that your submarine consists of only 1 asterisk which is located in row 8, column 1@. The computer displays the bomb at the random position specified by X and Y. Therefore, we can say, ‘If Y=8 AND X=1@, the submarine has been hit’.
In computer language, it becomes: IF Y=8 AND X=1@ THEN END AND
OO Now put five IF..THEN commands into the original program to detect whether any part of the submarine has been hit. Answer 31
OR
It is possible to shorten this program by incorporating all the IF..THEN commands into one line.
Let’s assume for a moment that your submarine consists of two asterisks, which are located in row 8, columns 1@ and 11.
IF (Y=8 AND X=19) OR (Y=8 AND X=11) THEN END
32
In this case, if either one of the brackets is true, the program will end. ie, if Y=8 and X=1@ OR 11, stop the program. OR
CW) Incorporate all five tests into one line of the submarine program, using the AND and OR commands.
WO Now that the computer can detect when the submarine has been hit, tell it to keep both player’s scores.
Number Games CL Write a program to do the following:
@ The computer randomly chooses four numbers (eg, 4,2,6,2) and displays them
on the screen. A fifth random number is 4262 then displayed below the four numbers (eg, 20). 20
You have to work out what mathematical operations need to be performed on the four numbers to produce the fifth number.
Note: you can use addition, subtraction, multiplication and division as many times as is required and you don’t have to use all of the numbers.
@ On pressing the spacebar, the computer displays a possible solution. 4262
20
S=A*B+C*D
For example, in our case, the computer displays S=A*B+C*D where:
A=1st Random Number B=2nd ...... ...
D=4th wu. S=5th Random Number
33
When you substitute the variables with their values, the equation looks like this:
4*2+6*2=29
Note: if you’re not sure how we arrived at the answer 29, look at the Appendix in Unit 1.
| vr How does the computer work out the equation? There are 2 possible methods:
The first method is by doing lots of AND and OR tests on the four random numbers but this is difficult and would take a long time to program.
The second method, which we have chosen to use, is easier, but it is cheating a bit. Instead of producing five random numbers, we produce four and work out the fifth from a random equation. The fifth number is then displayed on the screen.
The difference between the two is that in the first method, the equation is worked out by the computer (from the fifth number) whereas in the second method, the fifth number is worked out from the equation.
Now, before we write the program, lets see what you do or don’t know. You know how to display four random numbers on the screen, however, you don’t know how to randomly choose an equation. In order to do this, you need to learn a new command:
34
ON...GOTO
O Enter the following program into the computer and run itt:
19 CLS
20 INPUT R
39 ON R GOTO 40,50,60,79 ON...GOTO 49 PRINT "AAA":GOTO 20
o@ PRINT "BBB":GOTO 29
69 PRINT "CCC": GOTO 20
79 PRINT "DDD": GOTO 29
U) Input the numbers from 1 to 4.
Explanation
39 ON R GOTO 40,59, 60,79
This line tells the computer to go to a line in the program depending on the value of R. If R=1, the program will go to line 4Q, if R=2, the program will go to line 5@ etc. In fact, this one line replaces the four following IF...THEN commands:
1 THEN GOTO 49 2 THEN GOTO 59 3 THEN GOTO 69 4 THEN GOTO 79
IF R IF R IF R IF R
Task
Change the program in memory so that when
you press the number ‘2’, the computer
displays ‘CCC’, and when you press the
number ‘3’, the computer displays ‘BBB’. Answer 32
v7 What happens if you press ‘5’ or ‘9’?
The computer reacts in the same way as if you pressed the number ‘1’.
35
WW) Add the following line to the program:
35 PRINT "XXX":GOTO 20
C Now run the program and see what happens when you press a number greater than 4.
The computer ignores line 3@ and goes onto line 35.
Conclusion
If you input a value that does not have a corresponding line number, the computer will do one of two things:
e Firstly, it will look to see if there is a line after the ON...GOTO line and if there is, and providing this line is not specified in the ON...GOTO command, it will execute it.
elf the first check is not successful, the program will continue running from the first line number specified in the ON...GOTO command.
Task
Alter the program in memory so that the
computer randomly chooses a number
between 1 and 4 instead of the user inputing a
number. Answer 33
Back to the Number Game
You should now be able to write the Number Game that we described on page 32. If you have any difficulties writing this program, follow the stages below:
Note: remember that we changed the program so that the four random numbers produced are put into a random equation to produce the fifth number.
36
Stage 1
The first stage is to randomly choose a value (1-9) for the variables A, B, C and D and then to apply a random equation to the variables. Below are some equations that you can use, however, the more equations you have, the harder the game.
119 S=A*B+C-D:PRINT S:GOTO 189 120 S=A+B+C-D:PRINT S:GOTO 189 130 S=A/B+C*D:PRINT S:GOTO 180 Answer 34
Note: the program will not run until all the stages have been added.
Stage 2
At this point, if the user presses the spacebar, the solution is displayed.
The program lines to display the solution should be similar to those that define the fifth number. ie. lines 110-130
219 PRINT "S=A*B+C-D": GOTO 390 229 PRINT "S=A+B+C-D": GOTO 300 239 PRINT "S=A/B+C*D": GOTO 300
Note the order of the solutions. You can see
that it is the same as the order of the
equations. This means that you can use the
variable R again, but this time to display the
solution. Answer 35
ON R GOTO 219,229,230
You may well need to use the computer to work out the solution. If you do, stop the program running by pressing the ESC key twice and when you have finished working it out, type in the following command:
CONT
When you press ENTER, the computer CONTinues to run the program from the point CONT that you stopped it at.
37
Improving the Program You may have noticed that the fifth number is
not always a whole number.
CW Change the program so that when this occurs, another set of random numbers is displayed.
Hint: use the INT command. Answer 36
You could also change the program so that ifa negative number is displayed, it is treated in the same way as numbers with fractions. Answer 37
Summary We will end this chapter by asking you a few
questions about it.
t5) & The RND command tells the computer to choose a number between and
t5) ~ What would be displayed if you typed in the following line:
PRINT INT(Q@.9)
& The following command:
ON 3 GOTO 3,309, 50, 1900
causes the computer to continue execution from line
= v, The command tells the computer to continue running the program.
38
39
Chapter 4
A HORSERACE GAME
In this chapter, we will develop a ‘Horserace’ game:
Horses 1, 2 and 3 are in a race together. The first horse to reach the (specified) ‘line’ is the winner.
The computer randomly chooses a number between 1 and 3 inclusively. These numbers represent the horses in the race. Therefore, if the number 3 is chosen, horse 3 must be moved one position to the right.
It is the first horse to reach the ‘line’ that wins the race. In our example, horse 1 has won the race because the number 1 was chosen more times than 2 or 3.
Below is a listing of our solution to the problem, however, it has got a few ‘bugs’ in it.
18 MODE 0
15 X1=1:X2=1:X3=1
20 R=INT(RND*3)+1
25 IF Xl OR X2 OR X3=21 THEN END 30 ON R GOTO 50,69, 70
58 LOCATE X1,5:PRINT "1": X1=X1+]1
69 LOCATE X2,19:PRINT "2": X2=X2+]
79 LOCATE X3,15:PRINT "3": X3=X3+1
199 GOTO 20
1111
333
11111111111 222222 3333333
40
CC) Type the program into the computer and run it.
5) & Where is the first bug and how do we overcome it? Answer 38
When you have corrected the first bug, run the program a few times.
You can see that the outcome is always the same. Horse 3 wins followed by horse 2 and finally, horse 1.
C Change the program so that the outcome is always different. Answer 39
Improvements
e Draw a finishing line on the screen. 111
2222222
33333 |
@ Modify the program so that the computer asks you which horse you want to bet on.
DIM
At present, there are only three horses in the race, however, if you wanted to add more horses to make the race more exciting, you would end up copying a lot of the existing lines.
It is possible to add more horses to the race
and at the same time, shorten the program.
In order to do this, you must learn a new command: the DIM command. DIM
41
CC) Save the program currently in memory and type in the following line:
DIM A(20) It appears that nothing has happened, but, inside the computer, 20 memory locations have been DIMensioned (defined); A(®) to A(2Q).
At this point, they all contain the value of @.
Note: although the first location defined is Q,
in our case A(Q), we will not use it. The reason
for this will become apparent as you get
familiar with the use of the DIM command. Arrays
A group of memory locations that have the same name (A) is called an ARRAY.
Obviously, each location (element) must have a unique name or it would be impossible to access each individual element. Therefore, each element in an array is given a number (Subscript) that makes it different from the other elements.
A(2Q) Array Name iil “ Subscript
O Tell the computer to display the value of A(12).
—
The computer displays the number eee: fy Fy
42
? What would happen if you displayed the value of A(25)?
The computer's response would be to display:
Subscript out of range
All the computer is trying to tell you is that there isn’t a element with a subscript of 25 and as you will remember, we only dimensioned 20 elements.
A Limit on DIM
) ry What would happen if you dimensioned an array with 100090 elements? Try it.
DIM Z2(19999)
The computer displays the following message:
Memory full
In other words, the computer doesn’t have enough room in memory for an array of this size.
S & What does the following program do:
18 DIM C(4)
20 FOR N=1 TO 4 39 INPUT C(N) 49 NEXT N
When you run the program, the computer asks you for the value of C(1). After inputing a number and pressing ENTER, the computer asks you for another value, C(2) and so on upto C(4).
Note the way that we used the variable N to access each element in the array.
43
C) Now instruct the computer to display the contents of C(1). The number displayed should be the same as the first number that you input.
O Add lines to the program which will display the contents of each element in the array. Answer 40
OO Erase the computer’s memory and dimension the following array:
DIM Z(59)
Let’s assume that you want to enlarge the array to have 100 elements. You would probably type in:
DIM Z(199)
However, on pressing ENTER, the computer displays an error message:
Array already dimensioned
Conclusion
The computer cannot dimension an array with the same name, twice.
lf for some reason you need to change the size of an array, type in the following command:
CLEAR CLEAR
This command clears the computer’s memory of any variables or arrays that have previously been defined but will not erase the program in memory.
Note: there is no need to dimension an array that has less than 11 elements in it.
To prove this, type in the following commands:
PRINT G(9) PRINT G(11)
44
On executing the second command, the computer displays:
Subscript out of range
In other words, you tried to access an element in the array which has not been defined and as we have said, only 19 elements are given to an undimensioned array.
CO) Erase the computer’s memory and execute the following commands:
DIM A(7) DIM B(3Q) DIM C(500)
= t What does the computer display when you tell it to:
PRINT A(6) PRINT B(4Q) PRINT D(8) PRINT C(50 DIM A(13)
9)
Answer 41
CO) Type in and run the following program:
18 DIM C(15)
20 R=INT(RND*15) +1 39 C(R)=5
49 PRINT R
2 Which elements of the array equal @ and which equal 5?
Check your answer on the computer.
The program randomly chooses an element in the array and puts the value of 5 into it.
45
C5) { What will the program do if you change lines 30 and 49@ to the following:
39 C(R)=C(R)+5 49 GOTO 29
—*
y) £ Does anything appear on the screen when you run this program?
ae) ~ Does the program continuously run?
The computer continuously chooses an element in the array and adds 5 to it.
@ Stop the program running and display all the elements in the array using the following line:
FOR N=] TO 15:PRINT "C(";N; "y=""C(N) NEXT
You can see that each element has a different value depending on the amount of times it was randomly chosen.
Back to the Horseracing
You should now be able to shorten the horserace program described at the start of this chapter and also include the following improvements:
@® Increase the number of horses to 9.
@ Only one horse number is moved across the screen rather than a long line of numbers.
@ Stop the race when the first horse reaches the line.
46
Display the number of the horse that wins. Answer 42
When there are a number of people playing the game, the computer asks for each player’s name and the horse he/she wants to back. At the end of the race, the computer displays the number of the horse which wins and its backer.
The computer also displays the players names and the numbers of the horses which came 2nd and 3rd.
47
Chapter 5
STRING GAMES
A Word Recognition Game
We will start this chapter off by developing a ‘Word Recognition Game’ which should work as follows:
@ The computer stores a few words in memory, such as the following words:
RESOLUTION, INTERFACE, MICROPROCESSOR, SUBROUTINES, INPUT, DATA, READ, MEMORY PERIPHERAL, REGISTER, POINTER, HARDWARE, SOFTWARE
@ The computer then randomly chooses one of the words and displays it on the screen. You must try to spell the word correctly.
If you do so, the computer displays another word. If you don’t, the computer informs you of the error and displays the word again for a longer period of time.
Try to write this program on your own, however, if you should have any difficulties as we expect you will, continue reading.
String Arrays
The easiest way to display strings randomly is
by entering the words into a STRING ARRAY. String Arrays This allows us to randomly choose an element
in the array and thus, display the word.
48
String arrays work basically in the same way as numeric arrays. Below is an example of a string array:
DIM A$(3)
A$(1)="AMSTRAD" A$(2)="COMPUTER"
€ Astring array does not need to be dimensioned if it has less than 11 elements in it.
@ = The length of a string cannot be longer than 255 characters.
) v) In the above example, what is the value of AS(3). Answer 43
Putting the Words into the Array
As we said before, we must put the words into an array. There are a number of ways to do this and below is one way using INPUT.
CO Fill in the missing commands:
19 DIM W$(11)
20 FOR
39 INPUT
40 NEXT Answer 44
The problem with using INPUT is that everytime you want to use the program, you have to fill up the array manually, which is a long process.
To overcome this, you have to learn a new command:
49
READ...DATA READ...DATA
CO) Erase the computer’s memory and type in the following program:
18 READ X 20 a
Note how the numbers are separated.
O Run the program. It appears that nothing has happened but if
you print the value of X, you will see that something has.
v7 What did the computer do when you executed this program. When it executed the first line:
1 READ X
it set up a location in memory and called it X.
x |
On executing the second line, the computer copies the first piece of data (5) into location X.
READ X
29 DATA 5,6,7
el
O Now type in the following command:
READ X v7 What is the value of X now?
Check your answer on the computer.
50
Explanation
Within the program is a DATA statement which
holds the data that the program requires.
Before executing the READ command, the
Data Pointer points to the first piece of data, Data Pointer which, in this case, is 5.
20 DATA 5,6,7
Data Pointer
When the computer executes the READ command, it puts the value that the data pointer is pointing to, into X. ie. X=5. The data pointer is then automatically moved to the next item of data.
20 DATA 5,6,7 Data Pointer
Therefore, when the computer executed the READ command for the second time, X received the value of
t5)
{ What would the value of X be after instructing the computer to READ X for the third time?
C5) { What about the fourth time?
On the fourth time, the computer displays the following error message:
DATA exhausted
which means that there is no more data left, and as you can see from the program, there is only three items of data.
v7 What will appear on the screen when you run the following program:
18 FOR N=1 TO 4 29 READ X 39 PRINT X 49 NEXT N 59 DATA 1,3,8,13
CC) Change the program so that only the first two numbers (1 and 3) are displayed.
vr, What would happen if you put the DATA statement before the READ command? Try it.
Conclusion
The DATA statement can go anywhere in the program. When the computer executes a READ command, it takes the data from the first DATA statement that it finds in the program, however, it is usual to write the DATA statement at the end of the program.
) ¢ What will the computer display on the screen when you run this program?
19 READ X 20 READ Y 39 PRINT X/Y 49 DATA 19,2
Think about it first and then run the program.
Answer 45
51
52
Task
By adding the GOTO command, change the program in memory so that the computer displays the following equations with there respective answers:
ll / 17 =
19 / 37 = 34 / 83 = 96 / 33 = Answer 46
ty £ Did the computer display the ‘DATA exhausted’ error message?
We have already seen one way of overcoming this (using FOR...NEXT), however, below is another way:
19 READ X
20 READ Y
25 IF X=-1 THEN END
30 PRINT X;"/"-Y;"=""X/Y
35 GOTO 19
49 DATA 11,17,35,18,19,37,34
,83,96,33,-1,-1 65) & Can you explain why this program does not display the DATA exhausted message?
4 Is it necessary to have both ‘-1’ at the end of the DATA statement? Erase the last one and see what happens. Answer 47
Note
In the program above, there are two READ commands on lines 1@ and 29. It is possible to include both READ variables into one READ command as follows:
19 READ X,Y
53
The computer still takes two pieces of data from the DATA statement, where X equals the first piece (11) and Y equals the second piece (17).
On executing the READ command for a second time, the following two pieces of data (85 and 18) are put into X and Y respectively.
Task
There is a bug in the following program. Sort out what it is and correct the program accordingly.
19 FOR N=] TO 5
20 DIM X(5)
3@ READ X(N)
49 NEXT N
50 DATA 1,13,-5,7,9 Answer 48
vr) What happens when you run the following program?
18 READ A$ 29 PRINT A$ 39 GOTO 19 49 DATA PAUL, ELAINE, LISA, JOHN
Explanation
The READ and DATA commands can also handle strings. Line 1@ sets up a string variable (A$) and puts the first piece of data that it finds in the DATA statement (PAUL), into AS.
Note: you do not need to put quotation marks around the four strings in the DATA statement. As with the string INPUT command, the computer knows what type of data to expect by the variable used in the program.
? What would happen if you took the ‘$’ out of lines 19 and 2@ and then executed the program?
54
RESTORE
CC) After putting the ‘$’s back into the program, enter the following line:
25 IF A$="JOHN" THEN RESTORE You may remember that when we first executed the program, a ‘DATA exhausted’ message was displayed. Now, after adding line 25, the program continuously displays the strings in line 4@.
The RESTORE command returns the Data RESTORE Pointer to the first piece of data in the DATA statement. In our case, the following happens:
49 DATA PAUL ,ELAINE,LISA, JOHN
RESTORE
Now, when READ is executed again, the first piece of data is put into A$.
Task
Cc) List the program currently in memory.
Swap the RESTORE command in line 25 for another command, without changing the overall effect of the program. Answer 49
Conclusion
When you execute a RUN command, whether it be immediately or within a program, it moves the Data Pointer back to the first piece of data, as well as reseting variables etc.
55
Task
Write a program to do the following:
® The computer asks you to choose a number between 1 and 19.
e Let’s assume that you chose the number 6. On pressing ENTER, the the word ‘SIX’ is displayed. The computer then waits for you to input another number.
Thus, the computer displays the number input, in word form.
Hint
ty t What number is displayed when you run the following program:
18 FOR N=1 TO 3 29 READ A
3 NEXT N
49 PRINT A
50 DATA 7,6,5,4,3
Back to the Word Recognition Game
You should now be able to write the program that we described at the start of this chapter. Read over the description before attempting to write it.
Answer 50
Answer 51
56
Improvements
@ If you misspell the word twice, the computer displays the word again and waits for you to press a key before continuing.
@ If you spell the word correctly, the computer doesn’t bother displaying it again.
57
Chapter 6
A TREASURE HUNT
In this last chapter, we will develop a game that will use most of the commands taught during this unit.
@ Onrunning the program, the computer randomly displays 20 ‘mines’. After about five seconds, the mines disappear from the screen.
@ When the mines have disappeared from the screen, but not from memory, the treasure hunter (@) and the treasure ($) appear.
The aim of the game is to get the treasure hunter to the treasure, without stepping on any of the mines.
® The treasure hunter is only allowed to move to the right and down. If he steps on a mine, the computer displays ‘BOOM’.
BOOM! 5
You should be able to write this program with the knowledge acquired during this unit. We advise you to write this program stage by stage but should you have any problems with it, you can always look at the following hints.
58
Hint 1
The main problem in programming this game is keeping track of the mines. We need to do this so that if the treasure hunter steps on a mine, we can blow him up.
In order to do this, we need to dimension two arrays:
DIM X(28) DIM Y(28) (1) (1) X(2) Y(2) X(3) Y(3) (28) (28)
lst mine is located at X(1),Y(1). 2nd mine is located at X(2),Y(2). 3rd mine is located at X(3),Y(3). 20th mine is located at X(2@) ,Y(2Q).
The program to do this should look something like this:
5 MODE 9 18 DIM X(20) ,Y(20)
26 FOR N=1 TO 20
30 X(N)=INT(RND*29) +1
49 Y(N)=INT(RND*24)+1
50 LOCATE X(N),Y(N):PRINT "*" 60 NEXT N
Note the way we dimensioned two arrays in one line.
Hint 2
At some point in the program, the treasure hunter’s position needs to be compared with the positions of the mines.
(Use the FOR...NEXT command.)
59
Note: these hints are directed towards one solution. Obviously, there are other ways to construct this program and we will leave it up to you to choose.
The full program is listed in Answer 52. Answer 52
Improving the Program
@ When the treasure hunter steps ona mine, the computer produces an explosion.
e As you get nearer to the treasure, a rising pitch sound is produced.
@ When you reach the treasure, the computer plays a tune.
@ If the game is too easy, put an extra obstacle in the way such as a hidden wall, which has only one opening.
e If the game is still too easy, vary the position of the opening in the wall.
elf the player gets lost, he/she is allowed to display the obstacles (mines and wall) once only.
60
61
SUMMARY INDEX
Below is a list of the commands and techniques taught during this unit.
Go over each one of them and see if you can remember them. (Explanations are given on the pages stated in brackets.)
Video Pencil (7) OR (31) Greater Than (10) ON...GOTO (34) Less Than (10) CONT (36) INKEY$ (11) DIM (40) Joysticks (16) Arrays (41) JOY(Q) (16) CLEAR (43) Part LIST (25) String Arrays (47) RND (27) READ...DATA (49) INT (28) Data Pointer (50) AND (31) RESTORE (54)
To Conclude
Now that you have finished the third unit in this series and acquired a great deal more knowledge about the Amstrad, you may be interested in ‘Amstrad Unit 4’ which goes into high resolution graphics.
62
63
ANSWERS
Answer 1
19 CLS
29 INPUT A
39 IF A=1 THEN PRINT "ONE" 49 IF A=2 THEN PRINT "TWO" 58 IF A=3 THEN PRINT "THREE" 69 IF A=4 THEN PRINT "FOUR"
119 IF A=1@ THEN PRINT "TEN" 159 GOTO 20
Answer 2
18 CLS
29 INPUT A$
39 IF A$="P" THEN PRINT "PAUL" 49 IF A$="L" THEN LIST
58 IF A$="N" THEN NEW
199 GOTO 20
Answer 3
19 CLS
20 X=20:Y=20:LOCATE X,Y:PRINT "*" 30 INPUT A$
40 IF A$="K" THEN X=X+1
50 IF A$="J" THEN X=X-1
60 IF A$="I" THEN Y=Y-1
70 IF A$="M" THEN Y=Y+1
80 LOCATE X,Y:PRINT "*"
199 GOTO 30
64
Answer 4
45 IF X=41 THEN X=49 55 IF X= THEN X=1 F Y=@ THEN Y=1 F Y=2
6 THEN Y=25
65 I 13 I
Note: we have not restricted the movement of the pencil into the bottom right hand corner of the screen, even though this does cause the program to go wrong. This is because the tests to do this are rather more difficult than the tests we are currently doing.
Answer 5 The video pencil doesn’t move because the letters tested for are all uppercase, and when
you press CAPS LOCK, you input a lowercase letter.
Answer 6 Stop the program running and display the
contents of A$. As you can see, A$ equals nothing.
Answer 7
Add the following line:
35 SOUND 1,59
Answer 8
Add the following lines:
15 Bg="*" 29 X=20:Y=10:LOCATE X,Y:PRINT B$ 77 IF A$="t" THEN GOTO 159
89 LOCATE X,Y:PRINT B$
179 GOTO 39
65
Answer 9
t =249 4=24] +=242 —+=243
Answer 10
49 IF A$=CHR$(243) THEN X=X+1 50 IF A$=CHR$(242) THEN X=X+1 69 IF A$=CHR$(240) THEN Y=Y-1 79 IF A$=CHR$(241) THEN Y=Y+1
Answer 11
Add these lines:
79 IF A$="C" THEN GOTO 200
290 LOCATE 1,25: INPUT "COLOUR" ;C 219 PEN C
220 GOTO 39
Answer 12
Add the following lines: 35 IF A$="-" THEN K=1
37 IF A$="=" THEN K=9 99 IF K=1 THEN LOCATE X,Y:PRINT " "
Answer 13 1
66
Answer 14
If you press fire, the number 16 is displayed. If you move the joystick and at the same time, press fire, the direction value is added to the fire value (16).
For example, if you move the joystick to the right and press fire:
JOY (8) =8+16
which equals 24.
Answer 15
Add these lines:
49 IF JOY(@)=8 THEN X=X+1 58 IF JOY(@)=4 THEN X=X-1 69 IF JOY(@)=1 THEN Y=Y-1 79 IF JOY(9)=2 THEN Y=Y+1
Answer 16
19 CLS
29 LOCATE 35,24:PRINT "$" 39 FOR X=1 TO 40
49 LOCATE X,5:PRINT ">" 58 FOR T=1 TO 50:NEXT T 60 LOCATE X,5:PRINT "LJ" 79 NEXT X
290 GOTO 30
Answer 17
Add these lines:
43 AS=INKEY$ 45 IF A$="9" THEN GOTO 300
67
Answer 18
Add these lines:
300 FOR Y=23 TO 1 STEP -1l 310 LOCATE 35,Y:PRINT "t" 320 FOR T=1 TO 5Q:NEXT T 330 LOCATE 35,Y:PRINT "LJ" 349 NEXT Y
Answer 19
Delete line 5@ and add the following line:
55 SOUND 1,X+59,10
Answer 20
Add these lines: 315 SOUND 1,Y+50, 19
317 IF SQ(1)>127 THEN GOTO 317
Answer 21
Add this line:
358 GOTO 19
Answer 22
Add these lines:
15 LOCATE 1,1:PRINT N 345 N=N+]
Answer 23
IF X=35 THEN LET M=M+1
68
Answer 24
17 LOCATE 1,3:PRINT M 347 IF X=35 THEN M=M+1
Answer 25
Add the following line:
18 IF N=19 THEN END
Answer 26
18 PRINT RND*6
Answer 27
18 PRINT INT(RND*6)+1
Answer 28
19 CLS
20 FOR N=1 TO 13 39 PRINT INT(RND*3) 49 NEXT N
Answer 29
18 MODE 9
29 LOCATE INT(RND*29) +1, INT( RND*25) +1
39 PEN INT(RND*16)
49 PRINT "*"
58 SOUND 1, INT(RND*209) +59 190 GOTO 26
69
Answer 30
19 CLS
20 LOCATE 29,19:PRINT "*****" 39 INPUT A
49 FOR N=1 TO A
58 X=INT(RND*49)+1
69 Y=INT(RND*25)+1
79 LOCATE X,Y:PRINT "0";
190 NEXT N
Answer 31
Add the following lines:
75 IF X=2@ AND Y=19 THEN END
77 IF X=21 AND Y=19 THEN END
80 IF X=22 AND Y=19 THEN END
82 IF X=23 AND Y=19 THEN END
84 IF X=24 AND Y=19 THEN END Answer 32
Change the following line:
30 ON R GOTO 49,60,50,70
Answer 33
20 R=INT(RND*4)+1
70
Answer 34
19 CLS
26 A=INT(RND*9)+1
30 B=INT(RND*9)+1
49 C=INT(RND*9)+1
50 D=INT(RND*9) +1
60 R=INT(RND*3)+1
7@ LOCATE 15,10:PRINT A;"L_]";B; nf ju ‘¢: "CJ" <p
80 LOCATE 21,12
98 ON R GOTO 119,120,139
119 S=A*B+C-D: PRINT S:GOTO 180 12@ S=A+B+C-D: PRINT S:GOTO 189 130 S=A/B+C*D:PRINT S$:GOTO 180
Answer 35
Add the following lines:
189 AS=INKEY$
199 IF A$<>" " THEN GOTO 189 2909 ON R GOTO 219,226,230
219 PRINT "S=A*B+C-D":GOTO 300 229 PRINT "S=A+B+C-D":GOT0O 300 230 PRINT "S=A/B+C*D":GOTO 309 300 AS=INKEY$
319 IF A$="" THEN GOTO 300
328 GOTO 19
Answer 36
Add the following lines:
189 IF INT(S)<>S THEN GOTO 10 185 A$=INKEY$
Answer 37
Add this line:
182 IF S<@ THEN GOTO 19
71
Answer 38
The first bug is in line 25. It should read as follows:
25 IF X1=21 OR X2=21 OR X3=21 THEN END
Answer 39
The reason why horse 3 always wins is because line 7@ is always executed. For example, the random horse chosen is 2. The computer executes line 6M as expected but when it finishes, it executes line 7@ as well. ie. both horse 2 and 3 are moved.
5@ LOCATE X1,5:PRINT "1": X1=X1+1:G0T0 29
60 LOCATE X2,10:PRINT "2": X2=X2+1:G0T0 29
70 LOCATE X3,15:PRINT "3": X3=X3+1:G0TO 20
And you can of course, delete line 109.
Answer 490
Add the following lines:
149 FOR N=1 TO 4 150 PRINT "C(";N;")=";C(N) 160 NEXT N
Answer 41 Displaying the contents of A(6) will produce @.
Displaying the contents of B(4@) will produce a ‘Subscript out of range’ error.
Displaying the contents of D(8) will produce @.
72
Displaying the contents of C(5Q@) will produce 0.
Executing DIM A(13) will produce an ‘Array already dimensioned’ error.
Answer 42
18 MODE 2
20 FOR I=1 TO 9
39 X(1)=1
49 NEXT I
58 R=INT(RND*9)+1
69 LOCATE X(R),2*R:PRINT R: X(R)=X(R)+1
79 IF X(R)=79 THEN GOTO 200 89 GOTO 59
209 LOCATE 1,29:PRINT "THE WINNER IS HORSE.";R
Answer 43
A$(3) has not been defined. It therefore has a value of “” (nothing)
Answer 44 19 DIM W$(11) 20 FOR N=1 TO 11
39 INPUT W$ 40 NEXT N
Answer 45
Change this line:
19 FOR N=1 TO 2
73
Answer 46
Add the following lines:
SH PRINT Kotha SN/Y 35 GOTO 19
49 DATA 11,17,35,18,19,37, 34,83, 96, 33
Answer 47
It is necessary to have both ‘-1’s in the DATA statement because the first -1 is not tested for until both READ command have been executed.
Answer 48
The bug is caused by putting DIM X(5) into the loop. As we have already said, an array with the same name cannot be dimensioned twice.
To correct the program, move line 2@ to line 5.
Answer 49
25 IF A$="JOHN" THEN RUN
Answer 5@
19 CLS
20 INPUT A
39 FOR N=1 TO A
49 READ A$
5@ NEXT N
60 PRINT A$
79 RESTORE: GOTO 29
190 DATA ONE, TWO, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT ,NINE, TEN
74
Answer 51
19 2=59
20 CLS
39 R=INT(RND*5)+1
49 FOR N=1 TO R
59 READ A$
69 NEXT N
79 LOCATE 15,10:PRINT A$ 89 FOR T=1 TO Z:NEXT T 99 CLS
199 INPUT B$
119 IF A$=B$ THEN RUN 120 SOUND 1,209
139 Z=2+200
148 RESTORE: GOTO 40
158 DATA INPUT,DATA,READ, MEMORY , POINTER
We will leave it to you to add the rest of the words. Remember that line 3@ will also have to change depending on the amount of words you have.
Answer 52
18 MODE §:DIM X(2@) ,Y(2)
38 FOR N=1 TO 20
49 X(N)=INT(RND*29)+1:Y(N)= INT(RND*24) +1
69 LOCATE X(N),Y(N):PRINT "*" 80 NEXT N
99 FOR T=1 TO 3009:NEXT T:CLS 199 PRINT CHR$(143)
11Q LOCATE 19,23:PRINT "$" 120 X=1:Y=1
159 AS=INKEY$:IF A$="" THEN GOTO 159
169 IF A$=CHR$(243) THEN X=X+1:IF X=21 THEN X=20
170 IF A$=CHR$(241) THEN Y=Y+1:IF Y=24 THEN Y=23
188 LOCATE X,Y:PRINT CHR$(143)
200 FOR N=1 TO 20
219 IF X(N)=X AND Y(N)=Y THEN GOTO 490
220 NEXT N:GOTO 159
490 LOCATE X,Y:PRINT "BOOM!" 419 FOR A=1 TO 1900:NEXT A 429 RUN
75
NOTES
This series of self-instruction books will teach you the sec- rets of writing programs in BASIC on your AMSTRAD com- puter.
Unit 1: FIRST STEPS IN BASIC
Starting with the first things every programmer need to dale\semnvZeleM sim (=t-1e am Comst10[-m ere) aalaat-lale\-mcom dal-more)anl oleic] @ MYM alelarem-laremaelalaliarem olgele|e-laal-em=) a tal-e-ale elm cals) Uiavian’Zele Mim el-m-]e)(-mcom pat-].¢-mZel0] meorelanlelejt-]mmol-laie]saamelci-1i0)| and interesting tasks.
Unit 2: EXPLORING BASIC
aM alt-muesaliamc:t-[eial-s-mmZe10 mm dali gale) aml aal ole) ac: [alam erelaler-] 0) (ame) BASIC: numeric variables, string variables, FOR... NEXT andIF... THEN statements, and much more. You'll create a fo ffelhes] mexelaal ele) «=i mel (olel.em- lave mianicia-t-1dlalemela-] o)allorm elaele|e-laats Tarei {Uke llarvem-laliaar-1ae)ar
Unit 3: COMPUTER GAMES
In this unit you will learn to develop various computer Wale [=Xome-]nalct-P atm ZOlUM olgeleia-1s-mdalcelelelamcal-melali omalc\ a e)gelen ie-Taalaaliare mexe)arer=)ele-M-10 [el al-l-me-larelelaamalelaalel-1e-M 711m el-mial agen fo [Ulot-re Mm =)van dal -m dale mZelUM Mm ar-h’Z-mere)aly(e(-1e-]e)(-m elgelele-laalaaliare skills.
Nite) a¢-) am dal: ) en MM Od T-Talaliale Masle)a-Melili cm come(-t-] MY da| special subjects, such as 3D graphics, machine code and more.
£3.95
ISBN 1-85181-005-6
sLe P © PUBLISHERS LIMITED Glentop Publishers Ltd., Standfast House, Bath Place, High Street, Barnet, Herts. ENS 1ED 9 "781851°810055
Tel: 01-441 4130
AMSTRAD 464,664 & 6128 COMPUTER GAMES WATSON’S NOTES Ve) Ee}
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